I think the instruments that give you temperature that is physical without melting/vaporizing are most reliable ones than these arbitrary numbers created by researchers. Even plasma is said to reach merely 2000-10 000+ degrees celsius.

I wouldn't be so sure they can sustain that kind of plasma "indefinitely" (if the definition of that word is "For a long time, no end defined", then yes) as they mention they want to achieve, you would always need some energy to keep it going on, it might take anti-matter from the cosmic side and that would make it long lasting, but I think it is in the wrong place here on Earth to work like that. It probably would need to be outer space to really work properly and out of the Moon's orbit.